1. Field of the Invention
The invention relates to a system for digitally processing a given number of analog channel signals each having a given bandwidth .DELTA.f, which system for converting the said baseband channel signals into a single-sideband frequency-division multiplex signal comprises in cascade:
(a) input circuits which are formed by converters for converting the digital signals which correspond to the analog signals to be converted into pairs of digital signals in which the codewords occur simultaneously at a frequency .DELTA.f,
(b) a Fourier transformation device which is provided with N pairs of inputs and with 2 N outputs, where N at least is equal to the number of channel signals to be converted, and which is connected to a coefficient source which supplies carrier signal functions,
(c) a first set of 2 N arithmetic units which is connected to a source of filter coefficients which are formed by the values of the impulse response of a low-pass filter having a cut-off frequency equal to .DELTA.f/2, which arithmetic units are in the form of digital filters which each use a given set of coefficients formed by the values of the said impulse response at instants which occur with a period 1/.DELTA.f and in each set are shifted with respect to a reference instant by a given multiple of 1/2N.DELTA.f,
(d) a series-parallel converter having 2 N inputs and one output at which a digital signal appears which corresponds to the desired multiplex signal.
Providing such a system with elements which perform reciprocal operations enables the reverse operation, i.e. the conversion of the said multiplex signal into baseband channel signals, to be performed,
2. Description of the Prior Art
This system is used particularly with respect to speech signals for performing, by digital means, operations for producing a frequency-division multiplex signal and for demultiplexing this multiplex signal. Such systems are described in our prior U.S. Pat. No. 3,891,803. As is set out in this patent, an important property of the prior art device is the low computing speed of all computing circuits, for they operate on digital signals in which the codewords occur at a frequency equal to the bandwidth .DELTA.f of a channel signal (4 kHz in the case of speech signals). This low computing speed greatly facilitates large-scale integration of the system.
While retaining this important advantage the present invention provides a new embodiment of the abovementioned system in order to reduce the equipment required and hence the cost.